In Pokémon Platinum, like with Diamond & Pearl, Mr. Backlot owns a mansion in Route 212. This mansion has got a garden with many wild Pokémon within, specifically Pichu, Pikachu, Roselia & Staravia. However, after you get the National Pokédex, even more Pokémon start to come to the garden.

Go inside the mansion and talk to Mr. Backlot within the eastern room of the mansion. If you have the National Dex, he will go on about how he has seen a specific Pokémon in his garden. His butler will say it doesn't exist in the garden and will then exit. Mr. Backlot insists he's seen it and then lets you go. He did in fact see the Pokémon as they are now in the garden. However, the Pokémon within the garden change each day depending on which one Mr. Backlot says. The Pokémon stay for two days so there should always be two rare Pokémon within the garden at any one time. The list is as follows

It should also be noted that the book on Mr. Backlot's desk is the only place in-game where you can get Manaphy counted as being seen in the Pokédex. While you can transfer and trade it over, it is not available in-game. This is necessary in order to get your National Pokédex so be sure to check